In semiconductor manufacture, micro lithography is used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light, is passed through a photomask or reticle and onto the semiconductor wafer. The photomask contains opaque and transparent regions formed in a predetermined pattern. A grating pattern, for example, may be used to define parallel spaced conducting lines on a semiconductor wafer. The ultraviolet light exposes the mask pattern on a layer of resist formed on the wafer. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. The patterned resist can then be used during a subsequent semiconductor fabrication process such as ion implantation or etching.
As microcircuit densities have increased, the size of the features of semiconductor devices have decreased to the sub micron level. These sub micron features may include the width and spacing of metal conducting lines or the size of various geometric features of active semiconductor devices. The requirement of sub micron features in semiconductor manufacture has necessitated the development of improved lithographic processes and systems. One such improved lithographic process is known as phase shift lithography.
With phase shift lithography the interference of light rays is used to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the object and allows resolutions as fine as 0.25 .mu.m to occur.
In general, a phase shifting photomask is constructed with a repetitive pattern formed of three distinct layers or areas. An opaque layer provides areas that allow no light transmission, a transparent layer provides areas which allow close to 100% of light to pass through and a phase shift layer provides areas which allow close to 100% of light to pass through but phase shifted 180 degrees from the light passing through the transparent areas. The transparent areas and phase shift areas are situated such that light rays diffracted through each area is canceled out in a darkened area there between. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the opaque layer of the mask on a photopatterned semiconductor wafer.
Recently, different techniques have been developed in the art for fabricating different types of phase shifting photomasks. One type of phase shifting mask, named after a pioneer researcher in the field, M. D. Levenson, is known in the art as a "Levenson" phase shifting mask. This type of mask is also referred to as an "alternating aperture" phase shifting mask because opaque light blockers alternate with light transmission apertures and every other aperture contains a phase shifter.
This type of mask is typically formed on a transparent quartz substrate. An opaque layer, formed of a material such as chromium, is deposited on the quartz substrate and etched with openings in a desired pattern. Phase shifting areas on the mask are formed by depositing a phase shifting material over the opaque layer and into every other opening in the opaque layer. This is termed an "additive" phase shifting mask. Alternately, phase shifting areas of the mask may be formed in areas of the quartz substrate having a decreased thickness. This is termed a "subtractive" phase shifting mask.
Two types of Levenson phase shifting photomasks are shown in FIGS. 1A and 1B. FIG. 1A shows an additive phase shifting mask 8 comprising a transparent substrate 10 and opaque light blockers 12 with light transmission openings 16 therebetween. The phase shifters 14 for the phase shifting mask 8 are formed as segments of a light transmissive material, such as SiO.sub.2, deposited in every other light transmissive opening 16.
FIG. 1B shows a subtractive phase shifting mask 8A. In a subtractive phase shifting mask 8A, the phase shifters 14A are formed by etching the substrate 10A aligned with every other light transmission opening 16A in the opaque layer 12A. In the subtractive phase shifting mask 8A, the unetched portions of the substrate 10A form the phase shifters 14A. Although the additive and subtractive phase shifting masks are fabricated by different methods, the operation of these photomasks is equivalent.
The operation of the phase shifting mask 8 contrasted with the operation of a conventional photomask 18 formed with a transparent substrate 20 and opaque light blockers 22, is shown in FIGS. 2A-2D. FIG. 2B shows the electric field on the mask 8. FIG. 2C shows the electric field on the wafer. FIG. 2D shows the intensity on the wafer.
As shown in FIG. 2B, the phase shifters 14 create a "-1" amplitude on the wafer. This effectively reduces the spatial frequency of the electric field so that it is less inhibited by the lens transfer function of the imaging system and forms a higher contrast amplitude image at the wafer plane. When the electric field is recorded by the photoresist, only the intensity that is proportional to the square of the electric field amplitude is recorded. This doubles the reduced spatial frequency and produces an image of much higher contrast. In addition to the reduction of spatial frequency, the electric field is forced to pass through zero to -1. Thus edge contrast is improved. Therefore, the alternating phase shifting system benefits from the reduction of spatial frequencies as well as the enhancement of edge contrast.
Another type of phase shifting photomask is known in the art as a chromeless phase shifting mask. In a chromeless phase shifting mask there are no opaque (e.g., chrome) areas. Rather the edges between the phase shift areas and light transmission areas on the mask form a pattern of dark lines on the wafer. This is shown in FIGS. 3A-3D. A chromeless phase shifting mask 24 includes a transparent substrate 26 with a raised phase shifting area 28. The raised phase shifting area 28 may be formed by an additive or a subtractive process.
In the chromeless phase shifting mask 24 the edge 30 of the raised phase shifting area 28 prints a narrow dark line on the wafer. The amplitude passing through zero insures a zero intensity. The dark line is extremely narrow and has a much higher contrast in comparison to a dark line produced by a narrow opaque object without phase shifting.
One problem that occurs with a chromeless phase shifting mask 24 is that the raised phase shifting area 28, viewed from above, also has a terminating edge at each end. Such a terminating edge will form an undesirable "stringer" on the wafer. This is shown in FIGS. 4A and 4B. FIG. 4A is a plan view of a portion of the chromeless phase shifting mask 24 shown in FIG. 3A. FIG. 4B is a plan view the photoresist pattern formed at the wafer. The raised phase shifting area 28 of the photomask 24 includes the edges 30 that produce dark lines or "runners" 30A (FIG. 4B) on the photoresist.
In a similar manner, a terminating edge 32 of the raised phase shifting area 28 produces a stringer 32A on the photoresist. This stringer 32A is not a part of the desired pattern, and if not corrected, may cause shorting in the completed semiconductor devices. As will be apparent to those skilled in the art depending on the layout of a desired wafer pattern, stringers may occur in numerous other situations. As an example, a chromeless phase shifting area may change directions and connect or disconnect with other features on a wafer pattern.
In view of the foregoing, there is a need in the art for improved methods for forming chromeless phase shifting photomasks. Accordingly, it is an object of the present invention to provide an improved method for forming chromeless phase shifting photomasks and for eliminating undesirable null formation at the terminating edges of chromeless phase shifters. It is a further object of the present invention to provide an improved method for forming chromeless phase shifting photomasks that is simple and compatible with auto-CAD layout techniques used in semiconductor manufacture.